Estimating an open circuit voltage (OCV) by use of a measured closed circuit voltage (CCV) so as to estimate a state of charge (SOC) by use of the open circuit voltage is a known method for estimating a state of charge in a battery.
However, for a secondary battery which requires a long time until polarization is dissipated, it is difficult to estimate a state of charge accurately from an open circuit voltage because the battery has a large hysteresis upon charging/discharging in its SOC-OCV characteristics. A secondary battery in which, for example, SiO (silicon monoxide) is used for a negative electrode is a known secondary battery requiring a long time until polarization is dissipated.
A remaining secondary-battery capacity calculating device which has a simple and compact configuration and is able to accurately detect a remaining capacity in a secondary battery in which a flat region of the charge/discharge voltage is large is a known technology for estimating a state of charge. The remaining capacity calculating device weights, according to a rate of change in charge/discharge voltage, a charge/discharge voltage corresponding to the remaining capacity by use of a first remaining capacity obtained on the basis of a charge/discharge voltage or a second remaining capacity obtained on the basis of a charge/discharge-current integration value. In other words, even if a characteristic curve of the charge/discharge voltage is flat, the characteristic curve of the charge/discharge voltage gains a slope as a result of at least weighting the charge/discharge voltage by use of the second remaining capacity obtained on the basis of a charge/discharge-current integration value, which permits obtaining of an accurate remaining capacity in a secondary battery.
Patent Document 1: Japanese Laid-open Patent Publication No. 2012-137408